The present invention relates to CATV (Cable Television) distribution systems that utilize optical or light wave communication links, in combination with conventional radio frequency (RF) communication links, for the distribution of broadband CATV signals. More specifically, the present invention relates to an attachment ("sidecar") to the outer casing of a component of an RF distribution system, which provides a sealed enclosure in which one or more individual optical fibers may be separated from a main optical fiber transmission medium, and a conduit through which the individual fibers may be passed for connection to components housed in the RF casing.
In a conventional CATV distribution system, as shown in FIG. 5, the origination point of all signals carried on the system is the "headend", which typically consists of an antenna system, signal-processing equipment, combining networks and other related equipment. Television signals from commercial broadcast stations are collected from large "off air" television antennas and satellite earth receiving stations and transmitted over coaxial cable to the signal-processing equipment. The signal processor includes a number of electronic components that perform a variety of functions, including amplification of the collected signals before transmission to the distribution network, filtering out unwanted signals, adjusting the output level or strength of the collected signals so that all signals that are carried have close to the same level, converting the collected signals to transmission channels that are optimized for application to the cable system, and converting UHF signals to VHF cable channels. Finally, the combining network groups the signals from each cable channel into a single output for connection to a single coaxial cable for distribution through the CATV network.
The portion of the distribution system into which the headend output signals ar fed is generally called the trunk system, which is designed for bulk transportation of multi-channeled broad band CATV signals throughout the area to be covered by the cable system. Because the cable system distribution area tends to be quite large, the trunk system typically consists of coaxial cable with a series or cascade of 20 or more trunk amplifiers installed at intervals along the cable. The trunk amplifiers are necessary to compensate for the inherent losses in signal strength (attenuation) caused by transmission via coaxial cable. The signals are equalized and amplified by the trunk amplifiers such that the cable losses preceding each amplifier equal the gain of the amplifier. In other words, each cable span-amplifier combination forms a "unity gain" system, so that when span loss and amplifier gain are algebraically summed, the net system gain equals zero. Consequently, if each cable span-amplifier combination conforms to the unity gain principal, the entire trunk system will exhibit zero net gain.
Bridging amplifiers are used to feed signals from the trunk system to the feeder system of distribution lines that bring the signals to the subscribers. Directional couplers and/or splitters are used to select a portion of the signal from a trunk amplifier to be fed to the bridger amplifier. The trunk cable continues to other bridging stations (distribution nodes), where the signals are routed through distribution lines to other subscribers.
Up to four distribution or feeder lines are fed from each bridging amplifier to the subscriber areas. The distribution lines are also made of coaxial cable, though usually of a smaller diameter than the trunk cable. The signals travel down the feeder lines to the location of the first subscriber or subscribers. At this point, it is necessary to tap the signal off of the feeder lines so that it can be delivered to the subscriber residences. Directional taps and multitaps are used to tap the signals off the distribution lines. When the distribution lines are sufficiently long that the signal drops below the desired operating level due to losses from the coaxial cable and tap devices, one or more line extender amplifiers may be installed to compensate for the signal attenuation. The output of the tap devices feed flexible coaxial cable drop lines into the subscriber homes.
The various components of the CATV distribution network are installed in a number of different physical environments. For example, the trunk system may have trunk cable laid in underground ducts, with distribution nodes interfacing the feeder system placed above ground on telephone poles. Because many of the components are exposed to potentially damaging conditions, such as moisture, excessive heat or cold, and dirt or other contaminants, suitable materials must be utilized to render the component housings or casings water-tight and pressure-tight. In addition, the casings must provide shielding against emissions of excessive RF interference from the internal electronic components and leakage of electromagnetic interference into the casings. A die-cast aluminum casing with conductive silicone gasket seals is one example of a casing having the required sealing and shielding properties.
As stated above, one of the problems in CATV distribution networks is signal loss or attenuation. Another problem is the introduction of noise and distortion to the television signal by the various components in the network. For example, in the trunk system, cascades of trunk amplifiers are necessary to maintain the television signals at a desired output level. However, each amplifier contributes some noise and distortion. Thus, the wider the area covered by the CATV system, the greater the number of amplifiers required and consequently, the greater the amount of noise and distortion added.
In order to limit signal loss while also minimizing the introduction of noise and distortion, light wave communication over optical fibers has been effectively employed between the distribution system headend and distribution nodes to distribute multi-channeled broadband CATV signals. Optical fibers have low signal attenuation in comparison to conventional coaxial cable. Thus, light can be transmitted for long distances over optical fiber without requiring amplifiers. Use of optical fiber transmission media allows the elimination of the 20 or more trunk amplifiers that are needed for conventional trunk coaxial cables connecting the headend to distribution nodes for subscribers, thereby eliminating the noise and distortion caused by such amplifiers.
Problems remain, however, in the distribution of the signals from the fiber to the individual subscribers. As distinguished from RF signals carried by coaxial cables, which can be split off a main coaxial cable using inexpensive passive taps for distribution to subscribers, optical signals are difficult to divide without signal loss and/or distortion.
One way of solving this problem is to convert the signals delivered by the optical trunk lines to electrical RF signals ("optoelectric conversion") at the various bridging stations. The optoelectric conversion generates a broadband RF signal, which is fed via bridging amplifiers to the feeder system of coaxial cable distribution lines, as described above.
In connecting an optical fiber, either to another optical fiber or to an electronic device, consideration must be given to minimizing losses arising from the connection. Such losses can arise from fiber misalignment, reflection of internal light at the fiber end, and even dirt in the connection. Another source of signal attenuation results from bending effects. When an optical fiber is bent, light being guided within the fiber core may strike the boundary between the core and its surrounding cladding at an angle too large for the light to be reflected back within the core, resulting in the loss of light from the fiber. Excessive bending can also cause breakage of optical fibers. Thus, each optical fiber has a minimum bend radius which indicates the degree of bending the fiber can undergo before experiencing signal loss or physical damage.
Accordingly, it is an object of this invention to provide an optical fiber sidecar for an optoelectric signal converter that provides an enclosure in which individual optical fibers may be pulled off a main fiber cable for connection to the optoelectric converter, from which the RF signal continues through the CATV distribution system along coaxial cable to the subscribers.
It is a further object of this invention to provide an optical fiber sidecar that allows optical fibers to be connected to an optoelectric converter without attenuation of the broadband signal due to bending losses.
It is still a further object of this invention to provide an optical fiber sidecar that is pressure-tight and water-tight and is shielded against electromagnetic interference leakage.
Preferably, the optical sidecar comprises a sealed enclosure fixedly attached to the housing of the signal conversion device (trunk housing) in a pressure-tight and waterproof relationship to protect the optical fibers from physical degradation. The optical sidecar may further be shielded against electromagnetic interference (EMI) to prevent the introduction of signal noise from external sources and to prevent RF signals carried by the system from being emitted. The optical sidecar further comprises a sealed conduit between the optical sidecar and the signal converter housing that preferably maintains a pressure- and water-tight and shielded relationship and through which the optical fibers are passed to the optoelectric converter.
In a preferred embodiment, the main fiber cable is passed through the optical sidecar, with the cable entrance and exit openings suitably sealed to maintain the optical sidecar enclosure and adjacent trunk housing in a sealed and shielded relationship. Within the enclosure, up to four individual fibers are separated from the main cable by cutting or other suitable techniques. The individual fibers are then connected to short lengths of optical fiber, commonly referred to as "pigtails", by fusion splicing, which is a welded, permanent connection of two fiber ends that yields very low signal losses. A plurality of splicing trays are preferably provided within the optical sidecar enclosure to position the spliced fibers such that the minimum bend radius of each fiber is not exceeded. The optical sidecar conduit, which may for example take the form of a throughbore or bushing-grommet combination, is made just large enough in diameter to allow the fiber pigtails to pass through into the optoelectric converter housing for connection to the signal conversion device. The fiber pigtails may be connected to the optoelectric converter by any suitable technique, such as fusion splicing to an optical fiber pigtail extending from the converter or low-loss fiber connector port mounted on the converter.